Tube bending apparatus and method



y 3, 1966 P. J. MILLER 3,248,920

TUBE BENDING APPARATUS AND METHOD Filed July 9, 1964 3 Sheets-Sheet 1 INVENTOR. Pz-raw. J: MILLER.

F5. BY

ATTORN EYS y 1966 P. J. MILLER 3,248,920

TUBE BENDING APPARATUS AND METHOD Filed July 9,, 1964 5 Sheets-Sheet 2 INVENTOR. 'PETER J. M LLER ATTokNEYs y 1966 P. J. MILLER 3,248,920

TUBE BENDING APPARATUS AND METHOD Filed July 9, 1964 3 Sheets-Sheet 5 ZZ E 5 25$ INVENTOR. 'DE'I'ER J. MILLER ATTQRN EYS United States Patent 3 248,920 -TUBE BENDING APPARATUS AND METHOD Peter J. Miller, Toledo, Ohio, assignor to Toledo Heater Company, Toledo, Ohio, a corporation of Ohio Filed July 9, 1964, Ser. No. 381,467 6 Claims. (Cl. 72-298) The present invention relates to metal bending apparatus; and more particularly to a method and apparatus for bending tubes and the like, and to the articles produced thereby.

This is a continuation-in-part of my co-pending application Serial Number 234,305, filed October 31, 1962.

An object of the present invention is the provision of a new and improved method for bending metal objects such as tubes and rods and the like which prevents undue drawing and distortion of the objects during the bending operation.

A more particular object of the present invention is the provision of a new and improved method and apparatus for bending tubular fittings and the like which prevents undue thinning out of the walls of the tubing during the bending operation.

Further objects and advantages of the present invention will become apparent to those skilled in the art to which it relates from the following description of several preferred embodiments described with reference to the accompanying drawings forming a part of this specification, and in which:

FIG. 1 is an isometric view showing top and bottom die assemblies which operate according to principles of the present invention to prevent undue thinning out of the side walls of tubular fittings during a bending operation;

FIG. 2 is a fragmentary cross-sectional view taken approximately on the line 2--2 of FIG. 7 to better illustrate the manner in which the die assemblies of FIG. 1 grip and hold a part to be bent during the actual bending operation;

FIG. 3 is a fragmentary sectional view taken approximately on the line 3-3 of FIG. 2 to illustrate a work piece or blank positioned in the bottom set of dies prior to an actual bending operation;

FIG. 4 is a view similar to FIG. 3 except that it shows the lower die assembly and work piece at the end of an actual bending operation;

FIG. 5 is a longitudinal cross-sectional view through a work piece or fitting that has been bent by means of the apparatus shown in FIGS. 1-4;

FIG. 6 is a side elevational view of a bending machine having portions broken away to better illustrate its operating mechanism;

FIG. 7 is a fragmentary cross-sectional view taken approximately on the line 77 of FIG. 6;

FIG. 8 is a cross-sectional view taken approximately on the line 8-8 of FIG. 11 to better illustrate the construction of another embodiment of a die assembly which operates according to certain principles of the present invention;

FIG. 9 is a cross-sectional view taken approximately on the line 9-9 of FIG. 8;

FIG. 10 is a sectional view similar to that of FIG. 8, but showing the die assembly and work piece at the end of an actual bending operation; and

FIG. 11 is an isometric view of the die assembly shown in FIGS. 8, 9, and 10 along with its actuating mechanism which produces the actual bending movement.

FIG. 1 of the drawing shows upper and lower die block assemblies 10 and 12 in a separated position to receive a blank prior to an actual bending operation. The upper and lower die block assemblies 10 and 12 are identical except that they are opposite hand relative to each other and, therefore, only one of the die blocks need be described in detail. Those portions of the upper die block 10 which correspond to similar portions of the lower die block 12 are designated by like reference numerals characterized further in that a prime mark is affixed thereto.

The lower die block assembly 12 generally comprises a pair of pie shaped dies 14 and 16 which are confined within the cylindrical surfaces 18 of a split die block retainer 20 to provide cam means for guiding motion of the dies in a manner assuring desired compression of the work pieces during bending. The die block receiving opening 18 has an enlarged diameter portion 22 adjacent its inner or lower end for the purpose of providing a shoulder 24 to retain the die blocks 14 and 16 within the opening 18. The die blocks 14 and 16 on the other hand have a lower enlarged diameter portion 26 forming an axially outwardly facing shoulder 28 which abuts the shoulder 24 and prevents the withdrawal of the die blocks 14 and 16 out of the retainer opening 18. In order to provide further hold down means, the die blocks 14 and 16 have centrally located cylindrical surfaces whose inner ends are enlarged to provide inwardly facing shoulders 34 and 36 respectively which engage the bottom of the head 38 of a hold down bolt 40 that extends through a central opening of the die retainer plate 20 and its supporting structure to hold the center of the die blocks 14 and 16 in place. The hold down bolt 40 is also provided with a stepped portion 42 just beneath the head 38 which stepped portion is of an axial length slightly longer than the small diameter sections of the cylindrical openings 30 and 32 so that its outwardly facing shoulder 44 abuts the top surface of the die block retainer 20 and prevents binding of the die blocks 14 and 16 during the relative rotation.

Inasmuch as relative rotation of the die blocks 14 and 16 only requires that one of the die blocks be moved, the die block 14 is shown pinned or dowled to the retainer member as at 46. The die block 16, of course, is free to rotate within the cylindrical surfaces 18 of the retainer 20 and will be caused to assume a normal position prior to a bending operation wherein its side surface 48 abuts the side surface 50 of the die block 14.

The die blocks 14 and 16 are provided with depressions or recesses 52 and 54 in their outer coplanar surfaces 56 and 58 which just receive the lower half of a straight work blank when the die blanks are in their normal'position above referred to. The recesses 52 and 54 are shaped to exactly correspond to the surfaces of the work blank and can be thought of as providing abutment surfaces which both contact the lateral side surfaces of the work blank and the endwise facing portions of the work blank for reasons which will later be apparent.

As previously indicated the top die assembly 10 is provided with parts and surfaces which exactly correspond with those of the bottom die assembly 12, so that then a straight work piece is positioned on the bottom surfaces and the top die assembly is brought down into position over the bottom assembly, the work piece will be totally confined. The upper die block 14 is secured against rotation to its assembly 20' in the same manner as indicated for the lower die block 14, and the die blocks 16 and 16' are caused to rotate together to perform the actual bending operation by means of a pair of dowels 60 and 62 which are adapted for a sliding fit into corresponding openings 64 and 66 in the lower die-block 16. To aid in centering of the top die block assembly 20 to the lower die block 20 when the assemblies are brought together, the upper die block 14' is also provided with a pair of dowel pins 68 and 70 which are adapted to be slidingly received in corresponding openings 72 and 74 in the die block 14.

In the beginning position of an actual bending operation the die blocks 14, 14' and 16, 16' will be in the beginning position shown in FIG. 3 of the drawings wherein the adjacent sides 48 and 50 of the die blocks are in engagement with each other and their recesses 52 and 54 are aligned to receive a straight work piece W. The die blocks 14 and 16 are pie shaped sections which subtend angles of slightly less than 135 each so that the die blocks 16, 16 are adapted to rotate for slightly over 90 relative to the die blocks 14, 14'. In the present instance it is intended that the work piece W be bent 90. Inasmuch as there always is a certain amount of spring back during any bending operation, the die blocks 14 and 16 must be made to subtend angles of slightly less than 135 and in the present instance the die blocks 14 and 16 are each caused to subtend angles of 132 /2. The actual bending operation is performed by rotating the die block 16 and 16' through an angle of 95 until the opposite surfaces 76 and 78 of the die blocks are brought together. When the rotational force on the die blocks 16, 16', is released there is approximately of spring back so that the die blocks assume the final position shown in FIG. 4.

Any suitable means can be used for moving the upper and lower die assemblies and 12 together and for producing relative rotation between the die blocks 14, 14' and the die blocks 16, 16'. FIGS. 6 and 7 of the drawing show one convenient mechanism by which these motions can be accomplished. The machine shown in FIG. 6 generally comprises a cast frame 80 having a horizontal bed 82, an upright 84, and a head portion 86 that is spaced over the base portion 82. The lower die assembly 12 is appropriately held by the base portion 82, while the upper die assembly 10 is suitably supported on the lower end of a reciprocating head 88 that is adapted to move in and out of the head portion 86 of the frame in alignment with the lower die assembly 12. The reciprocating head 88 can be actuated in any suitable manner, however, it is most conveniently moved by means of a toggle 90 whose center pin 92 is moved generally laterally by means of an air cylinder 94 and connecting link v96. The top end of the toggle 90 is, of course, pinned to the frame, while its lower end is pinned to the reciprocating head 88, so that reciprocation of its center pin 92 by the air cylinder 90 will move the toggle between its collapsed and extended positions during which time the die block assemblies 12 and 10 are moved between their separated and abutting positions.

Relative rotation of the die blocks 14, 14' and 16, 16 is produced by means of another air cylinder 98 that is housed within the base of the frame 80. Rotation of the die block 16 is produced by means of a pin 100 that is attached to the lower surface of the die block 16 and which extends through an arcuate opening 102 in the lower surface of the die block retainer 20. The lower end of the pin 100 is pivotally connected to a connecting rod 104 which in turn is connected to the piston 106 of the air cylinder 98 by means of which the die block 16 is rotated until its surface 78 comes in contact with the surface 76 of the die block 14. Thereafter the release of air from the cylinder 98 allows the die block to spring back slightly.

The machine shown in FIG. 6 is operated by admitting air to the left side of the air cylinder 94 to collapse the toggle 90 and thereby separate the upper die assembly 10 from the lower die assembly 12. The die blocks 14 and 14' will,- of course, be in alignment with each other as will the die blocks 16, 16, and each of the die blocks 16, 16' will be in the position shown in FIGS. 3 and 7. Thereafter a straight workpiece which is to be bent and which has preferably been filled with a soft bendable plastic or metal such as Cerrosafe is placed in the recesses 52 and 54, and air pressure is admitted to the right side of the air cylinder 94 to straighten out the toggle and bring the upper die block assembly 10 into firm engagement with the lower die block assembly 12. Once the die blocks are in firm engagement, air pressure is adoutside diameter of the tubular section.

mitted to the right side of the air cylinder 98. At this time, dowel pins 60 and 62 will be inserted in the openings 66 and 64 to key the die blocks 16, 16 together so that extension of the air cylinder 98 will swing the die blocks together to their fully rotated position. With full air pressure admitted to the right side air cylinder 98, the abutment surfaces 78 and 76 of the die blocks 16 and 14 respectively will be moved into firm engagement; and upon release of air pressure from the right side of air cylinder 98, the workpiece W will cause the die blocks 16, 16' to spring back slightly to the position shown in FIG. 4 of the drawings.

Thereafter air pressure is admitted to the left side of the cylinder 94 to collapse the toggle and lift the upper die block assembly 10 out of engagement with lower die block assembly 12 to permit the operator to remove the bent workpiece W from the lower die block assembly. Before inserting another workpiece into the lower die block assembly, its die block 16 must be rotated back to the position shown in FIG. 3 as must the die block 16' of the upper die block assembly 10. This can be done in any suitable manner, and in the embodiment shown in the drawing, it is intended that the operator will do this by hand. After the die blocks in the upper and lower assemblies are rotated back into the position shown in FIG. 3, another workpiece is inserted and the process is repeated. As previously indicated, FIG. 5 of the drawing shows the finished bent workpiece.

It has previously been indicated that the present invention places the workpiece under compression as the bending operation takes place. In order that this may more fully be appreciated, reference should now be had to FIGS. 3 and 4 of the drawings. FIG. 3 of the drawing makes it clear that the die block 16 rotates about a point that is positioned a distance r from the left hand flange portion located in the die block 14. It will also be clear that a similar bend takes place in the right hand portion of the workpiece that extends a distance r to the right side of the center of rotation O. The total length of the section that is to be bent, therefore, before the actual bending operation will be equal to 2 r. After the bending operation, however, the center line of the fitting is caused to be bent upon a radius equal to r, so that the total length of the bent portion after bending is equal to 2m divided by 4 or 1.57r. It will now be clear, therefore, that a foreshortening of the bending portion takes place during the actual bending operation which tends to compress or hold more of the fitting fibers in compression than would otherwise take place. This compression upon the fitting tends to prevent undue stretching and thinning out of the outside fiber wall of the fitting during the bending operation. In order that the fitting can be bent to a smooth radius, the outer edges of the recesses 52 and 54 are rounded off as at 108 and 110 to provide a smooth radius for the inside fiber after bending.

It will be apparent that the rounded portions 108 and 110 determine the extent of the bend as well as the final radius of the finished bend and that the method and apparatus of the invention can be used to bend tubes which do not have enlarged or flange sections. Where a short cylindrical section is desired between the bend section and the enlarged or flanged portions, if such exist, the die blocks 14, 14' and 16, 16', as the case may be, are provided with hemicyclindrical recesses which just receive and confine the section of the tube which is to remain cylindrical. The curved surfaces 108 and 110 must conform to the inside surface of the tube depending upon the center line radius which the finished bend is to have. The method and apparatus can be used to bend tubular sections to any desired radius, and has particular merit over prior art processes, where the finished centerline radius is to be less than about two times the It has still further merit where the piece to be bent has integrel enlarged or flanged portions thereon. Tubular sections can be bent 90 degrees to a centerline radius equal to or slightly less than the outside diameter of the tube with excellent results. The finished pieces so produced have side walls both towards the center of curvature from the centerline radius and outwardly from the centerline radius that are cold worked and are stronger, than are the corresponding portions of hot forged elbows. Bent sections of any angle less than 90 degrees, with or without tangent sections can, of course, also be made by the process and apparatus of the present invention.

In some instances it will be desirable to provide less compression than occurs when the die block 16 rotates about a point on the center line of the fitting as is shown in FIGS. 3 and 4. Where less compression is desired, the center of rotation may be displaced to the side of the center line of the unbent fitting that is located to the side to which the bending operation is to take place. If a greater compression is desired the center of rotation of the die block 16 can be caused to occur on a point that is positioned on the opposite side of the center line of the fitting from the direction of bend.

Still other embodiments of die assemblies can be produced for bending work pieces according to the principles of the present invention. In the embodiment shown in FIGS. 8-11, only the outer half of the workpiece is confined during the bending operation, and the movable die block is caused to move vertically; so that the bent workpiece can readily be picked out of the dies after the bending operation. The die assembly shown in FIGS. 8-l1 comprises a fixed die block 210 having a suitable recess 212 in its upper surface for containing the outer lower surface of one-half of a work piece. Die block 210 is fastened to the front side of two vertical side blocks 214 and 216 which are suitably milled out to define a space 218 therebetween that is generally cylindrical to provide an outer cylindrical camming surface 220 upon which the outer camming surface of the pie shaped movable die block 222 rides. The portions of the blocks 214, 216 and 220 which form the camming surfaces, therefore, provide cam means for moving the die blocks in a manner producing the desired compression of the workpieces. Each of the side blocks 214 and 216 has an annular groove or recess 224 and 226 respectively milled therein adjacent the outer cylindrical surface 220. The movable die block 220 has an annular flange 228 which projects from its opposite side surfaces into the grooves 224 and 226 to more adequately retain the movable die block in position during its arcuate movement. The upper surface of the movable die block 222 also contains a recess 230 that is shaped to receive the lower half of its end of a workpiece W, so that the workpiece W will be confined on its lateral side surfaces as well as its ends during an actual bending operation.

Movable die block 222 can be rotated from its position shown in FIG. 8 to its actuated position shown in FIG. 10 in any suitable manner, and is accomplished in the machine shown in FIG. 11 by means of an air cylinder 232. The lower end of the air cylinder 232 is suitably pivoted to the side block 216, while its piston rod 234 is in turn pivoted about a'pin 236 that passes through an arcuate opening 238 in the side block 216 and is suitably fiXed to the right side of the movable die block 222. Admissionof air into the lower end of the air cylinder 232, therefore, causes the piston rod 234 to force the pin 236 upwardly, and the outer cylindrical surface 240 of the movable die block bears against and rides around the cylindrical surface 220 until the piston reaches the end of its stroke wherein the die block 222 is in its upper position shown in FIG. 10.

The operation of the embodiment shown in FIGS. 811 is in some ways more simple than is the operation of the previously described embodiment in that it permits the operator to more readily remove the bent piece from the die blocks. Operation of the machine shown in FIG.

11 begins with the die block 222 being in its lower position shown in FIG. 8. The workpiece W is placed in the recesses 212 and 230, and air pressure is admitted to the lower end of air cylinder 232 to cause the air cylinder to produce the arcuate movement of the die block 222 previously described. When the air cylinder reaches the end of its stroke and the movable die block 222 is in the position generally shown in FIG. 10, further movement stops and the operator thereafter reverses the air flow to the air cylinder to put the pressure on the top side of the air cylinder and thereby cause a retraction of the piston rod 234. This swings the die block 222 back into its lower position shown in FIG. 8, and leaves the workpiece W standing upright in the die block 210. The operator merely reaches in to remove the workpiece W, and then puts a fresh workpiece in the surfaces 212 and 230 to repeat the cycle. The remarks concerning the geometry of the previously described embodiment are also applicable to the present embodiment. Once this geometry is understood it will be appreciated that the movement of the die block 222 produces a compression in the Workpiece by reason of the abutting forces that are placed upon the end surfaces 240 and 242 of the workpiece as well as upon the flange 244 to hold the workpiece in position in the dies even though its outer surface is not confined.

It will be apparent that the objects heretofore enumerated as Well as others have been accomplished and that there has been a method provided for bending workpieces, such as tubular sections, in which the workpiece is placed under compression during the bending operation to prevent undue thinning out or distortion of the side walls of the tubular piece. What is more there has been provided a simple and convenient mechanism for placing this method into actual operation.

While the invention has been described in considerable detail, I do not wish to be limited to the particular embodiments shown and described; and it is my intention to cover hereby all novel adaptations, modifications, and arrangements thereof which come within the practice of those skilled in the art to which the invention relates.

What I claim is:

1. Tube bending apparatus and the like comprising: first and second die means adapted to receive and rot-ate opposite end portions of a tubular workpiece, cam means mounting said first and second die means for relative arcuate movement along a predetermined path in a given plane from a beginning position of said die means to a rotated position, the adjacent and surfaces of said die means being shaped to enable relative arcuate movement between said beginning and rotated positions, each of said first and second die means having work confining surfaces for confining the sides of the workpiece that are generally parallel to the given plane of movement, and each of said first and second die means further having abutment surfaces firmly fixed thereto which abut opposite endwise facing surfaces of the workpiece, said cam means being contoured so that said relative arcuate path has a center of curvature falling radially outwardly of the centerline of the workpiece when in its finished rotated position, and whereby material laterally of the centerline of the workpiece and normal to the direction of bend moves in the direction of bend and is foreshortened.

2. Tube bending apparatus and the like comprising: first and second die means adapted to receive and rotate opposite end portions of a tubular workpiece, said first and second die means being pivotally supported about an axis positioned between said die meansfor producing relative arcuate movement of said die means in a given plane from a beginning position of said die means to a rotated position, the adjacent end surfaces of said die means being shaped to accommodate relative arcuate movement between said beginning and rotated positions, each of said first and second die means having work confining surfaces for confining the sides of the work-piece that are generally parallel to the given plane of movement, said die means having clearance with respect to the initial position of the side of the workpiece toward which said die means are rotated to allow said workpiece to move in the direction of the rotation and become foreshortened during rotation of said die means, said axis of said die means also being positioned radially outwardly of the centerline of the workpiece when in its finished rotated position, and each of said first and second die means further having abutment surfaces for abutting opposite endwise facing surfaces of the workpiece to produce compressive force on said workpiece during movement of said first and second die means between said beginning and rotated positions.

3. Tube bending apparatus and the like comprising: first and second die means adapted to receive and rotate opposite end portions of a tubular workpiece, said first and second die means being pivotally supported about an axis positioned between said die means for producing relative arcuate movement of said die means in a given plane from a beginning position of said die means to a rotated position, the adjacent end surfaces of said die means being shaped to accommodate relative arcuate movement between said beginning and rotated positions, each of said first and second die means having work confining surfaces for confining the sides of the workpiece that are generally parallel to said given plane of movement, and each of said first and second die means further having abutment surfaces for abutting opposite endwise facing surfaces of the workpiece to produce compressive force on said workpiece during movement of said first and second die clamping means between said normal and rotated positions, said die means being so arranged that their pivotal axes falls approximately on the center line of the work confining surfaces when in their beginning position.

4. Tube bending apparatus and the like comprising: first and second die means adapted to receive and rotate opposite end portions of a tubular workpiece, means mounting said first and second die means for relative arcuate movement along a predetermined path in a given plane from a beginning position of said die means to a rotated position, the adjacent end surfaces of said die means being shaped to accommodate relative arcuate movement between said beginning and rotated positions, each of said first and second die means having clamping surfaces for gripping its respective end of the workpiece and for confining [the sides of the workpiece that are generally parallel to said given plane of movement, said die means having clearance with respect to the beginning position of the side of the workpiece toward which said die means are moved to allow said workpiece to move in the direction of bend and become foreshortened during said arcuate movement of said die means, said path having a center of curvature positioned radially outwardly of the centerline of the workpiece when in its rotated position, each of said first and second die means further having clamping surfaces for abutting opposite endwise facing surfaces of the workpiece, and the arcuate path being such that the centerline of the workpiece is subjected to a longitudinal compressive force during movement of said first and second die means between said beginning and rotated positions, and each of said first and second die means being separable on a plane parallel to said given plane and passing through their clamping surfaces.

5. Tube bending apparatus and the like for forming a bend at a predetermined location in tubular workpieces, said apparatus comprising: first and second die members having adjacent side surfaces normally arranged side by side and having upper surfaces which lie in a common plane when said die members are in their normal side by side relationship, said first and second die members each having abutment surfaces adapted to receive respective end portions of a workpiece when said predetermined location of the workpiece is positioned over the adjacent side surfaces of said die members, and rotate said end portions between beginning and rotated positions, means for moving one of said die members relative to the other die member arcuately upwardly in a path whose center of curvature is located generally in the region between said adjacent side surfaces of said first and second die members and radially outwardly of the centerline of a workpiece when in said rotated position, each of said abutment surfaces having a portion fixed thereto and facing said adjacent side surfaces of said die members and which abut opposite end facing surfaces of the workpiece, and whereby said arcuate path of relative movement of said die members compresses the portions of the workpiece that are disposed laterally of the centerline during said relative movement.

6. Tube bending apparatus and the like for forming a bend at a predetermined location in tubular workpieces, said apparatus comprising: first and second die members having adjacent side surfaces normally arranged side by side and having upper surfaces which lie in a common piane when said die members are in their normal side by side relationship, said first and second die members each having abutment surfaces adapted to receive respective end portions of a workpiece when said predetermined location of the workpiece is positioned over the adjacent side surfaces of said die members and rotate said end portions between beginning and rotated positions, means for moving one of said die members relative to the other die member arcuately upwardly in a path whose center of curvature is located generally in the region between said adjacent side surfaces of said first and second die members and radially outwardly of the centerline of a workpiece when in said rotated position, each of said abutment surfaces having a portion facing said adjacent side surfaces of said die members and which abut opposite end facing surfaces of the workpiece, said abutment surfaces also having portions which confine the side surfaces of a workpiece that are disposed laterally relative to said direction of relative movement, and whereby said arcuate path of relative movement of said die members compresses the portions of the workpiece that are disposed laterally of the centerline during said relative movement.

References Cited by the Examiner UNITED STATES PATENTS 1,931,306 10/1933 Taylor 29157 2,041,365 5/1936 Mitchell et a1 15340 2,155,079 4/ 1939 Cornell 29157 72,303,949 12/1942 Nordell 13839 2,303,950 12/1942 Nordell 138-39 3,017,914 1/1962 Fuchs 15340 CHARLES W. LANHAM, Primary Examiner.

R. D. GREFE, Assistant Examiner. 

1. TUBE BENDING APPARATUS AND THE LIKE COMPRISING: FIRST AND SECOND DIE MEANS ADAPTED O RECEIVE AND ROTATE OPPOSITE END PORTIONS OF A TUBULAR WORKPIECE, CAM MEANS MOUNTING SAID FIRST AND SECOND DIE MEANS FOR RELATIVE ARCUATE MOVEMENT ALONG A PREDETERMINED PATH IN GIVEN PLANE FROM A BEGINNING POSITION OF SAID DIE MEANS TO A ROTATION POSITION, THE ADJACENT AND SURFACES OF SAID DIE MEANS BEING SHAPED TO ENABLE RELATIVE ARCUATE MOVEMENT BETWEEN SAID BEGINNING SAID ROTATED POSITIONS, EACH OF SAID FIRST AND SECOND DIE MEANS HAVING WORK CONFINING SURFACES FOR CONFINING THE SIDES OF THE WORKPIECE THAT ARE GENERALLY PARALLEL TO THE GIVEN PLANE FOR MOVEMENT, AND EACH OF SAID FIRST AND SECOND DIE MEANS FURTHER HAVING ABUTMENT SURFACES FIRMLY FIXED THERETO WHICH ABUT OPPOSITE ENDWISE FACING SURFACES OF THE WORKPIECE, SAID CAM MEANS BEING CONTOURED SO THAT SAID RELATIVE ARCUATE PATH HAS A CENTER OF CURVATURE FALLING RADIALLY OUTWARDLY OF THE CENTERLINE OF THE WORKPIECE WHEN IN ITS FINISHED ROTATED POSITION, AND WHEREBY MATERIAL LATERALLY OF THE CENTERLINE OF THE WORKPIECE AND NORMAL TO THE DIRECTION OF BEND MOVES IN THE DIRECTION OF BEND AND IS FORESHORTENED. 